Never been a fan, towed thousands of miles without them. Never had a problem.
However, my wife is about to do her towing course, so a bit of practice with L plates and our twin axle monster, I thought I'd better invest in a decent pair for her.
I tow with a Range Rover, so have a better than most field of vision anyway. No need to get all holier than thou, I started the thread for mirror recommendations.